As those skilled in the art know, in certain fields laser sources are used that deliver a beam of photons having wavelengths that are distributed substantially continuously over a large spectral width, typically a few tens of nanometers to a few hundred nanometers. This is in particular the case in the field of analysis of samples (possibly medical samples).
These polychromatic sources, which are frequently called continua, generally result from a light-matter interaction involving nonlinear effects. They often comprise at least one pulsed laser source that delivers “primary” photons having a “primary” wavelength, and an optical fiber that is micro-structured in order to produce, from the primary photons, an output beam containing secondary photons having a number of “secondary” wavelengths (that together form a supercontinuum).
A non-linear micro-structured optical fiber is generally made of silica and comprises micro-structures that are intended to confine the light power in order to increase the light-matter interaction and thus improve the conversion of the primary wavelength to many secondary wavelengths. By way of example, these micro-structures may form a Bragg grating that is transverse to the propagation direction of the light in the optical fiber and that is able to modify the dispersion relationship seen by the light.
By virtue of this type of source, i.e. sources that employ one or more micro-structured optical fibers, it is possible to obtain stable emissions with a spectral width ranging from the near ultraviolet (or UV) (about 350 nm) to the mid infrared (typically 5 μm). The performance of micro-structured optical fibers made of silica is for example limited in the infrared beyond about 2.4 μm.
Unfortunately, the core of these micro-structured optical fibers is of small diameter. Therefore, when energy is highly confined thereto, the threshold for damage to their core material is very quickly reached, and therefore sources employing them cannot deliver high output energies. Moreover, when these microstructured optical fibers are pumped in the normal dispersion regime, this induces a non-continuous generation of the conversion spectrum via the stimulated Raman effect, and hence sources employing them cannot be considered to be veritable continua.
As a result these sources cannot be used in certain applications, such as for example multiplex coherent anti-Stokes Raman scattering (CARS) microspectroscopy, because of the spectral non-continuity of the emitted radiation. It will be recalled that multiplex CARS microspectroscopy is in particular used in the field of imaging and of spectroscopy to identify and locate specific chemical species within a sample or in an open space.
When it is desired to generate a supercontinuum with a very high energy, an optical fiber having a core diameter that is notably larger than that of a single-mode or quasi-single-mode micro-structured optical fiber must be used. To this end, a multimode optical fiber may be used.
The latter type of optical fiber allows higher energies to be guided but does not have the capacity to greatly modify the dispersion of the guide because of its large core width. The radiation output is equally distributed between a plurality of modes, this greatly decreasing the brightness of the output radiation.
A supercontinuum may be created in optical fibers by virtue, in particular, of a mix of the Raman effect and of parametric processes. Because of the Raman effect and of the dispersion regime, the secondary photons of the supercontinuum are generated at wavelengths that are above the primary (or pump) wavelength, this making it difficult to generate secondary wavelengths in a spectral domain below that of the pump wave. For an excitation wavelength located in the normal dispersion regime, the wavelengths of the supercontinuum are generated in packets because of the non-continuous profile of the Raman gain, this preventing a veritable spectral continuity from being achieved. Despite the partial modal filtering induced by the Raman conversion, the output radiation is also obtained in many modes, this decreasing the brightness and spatial coherence of the source.